In recent years, with the prevalence of portable electronic devices, such as mobile phones and digital cameras, technological development of imaging modules having an optical lens, a module holder unit and a sensor has thrived. Generally, the optical lens of such imaging modules is designed to be an imaging zoom lens that allows a user to change a focal length thereof in a predetermined range for satisfying photographing requirements of the user. In addition, the imaging zoom lens can magnify a distant object for taking a close-up photograph when zooming at a telephoto end, and can have an enlarged field of view when zooming at a wide angle end.
Since a focusing range of travel of the imaging zoom lens from infinity to a close-up photographing distance is proportional to a system focal length of the imaging zoom lens, the imaging zoom lens at the telephoto end, which has a maximum system focal length, leads to a maximum focusing range of travel from infinity to the close-up photographing distance, thereby causing a slow focusing speed. Hence, it is a goal in industry to increase the focusing speed at the telephoto end to approach the focusing speed at the wide angle end.